Translucent, room temperature curable silicone sealants frequently demonstrate adequate auto-adhesion to glass, aluminum and PVC. Adhesion to other plastics, for example polycarbonate, is often less than desired and frequently degenerates upon prolonged exposure to water. For example, translucent, alkoxycuring RTV compositions disclosed by Lucas et al. (U.S. Pat. Nos. 4,483,973 and 4,528,353) show excellent long-term primerless adhesion to glass, aluminum, and PVC, and demonstrate excellent short-term (i.e. &lt;3 month at room temperature) adhesion to polycarbonate. However, the RTV compositions utilize di-amino functional silane adhesion promoters which chemically attack the polycarbonate surface causing crazing and adhesion loss within 1-3 months.
The use of titanium chelates as self bonding agents for alkoxy curing RTV's is disclosed by Lucas (U.S. Pat. No. 4,680,364). The compositions are fumed silica filled and are self bonding to polycarbonate but have the limitation of opacity due to the poor solubility of the titanium chelates in PDMS.
There are numerous other examples in the art describing self bonding, 1-part RTV compositions for example; Ashby and Lucas (U.S. Pat. No. 4,506,058), Beers (U.S. Pat. No. 4,100,129 and 4,514,529) Mitchell (U.S. Pat. No. 4,273,698), composition having improved primerless adhesion to polycarbonate, comprising:
(A) 100 parts of a polyalkoxy terminated PDMS having the formula 1: ##STR1## where each R and R.sup.2 is independently a substituted or unsubstituted monovalent hydrocarbon radical having from 1 to about 15 carbon atoms C(.sub.1-15), R.sup.1 is a C.sub.(1-8) aliphatic organic radical selected from alkyl radicals, alkyl ether radicals, alkylketone radicals, alkylcyano radicals having from 1 to about 8 carbon atoms C.sub.(1-8) , or an aralkyl radical having from about 7 to about 13 carbon atoms C.sub.(7-13) ; "n" is a whole number ranging from about 5 to about 2500, and "a" is a whole number which is either 0 or 1. The viscosity range of the polymer of formula (1) is 50 to about 65,000 cps at 25.degree. C.
(B) from about 5 to about 20 parts by weight based upon (A) of a silica filler, preferably fumed silica having a surface area up to about 300 m.sup.2 /gm. in the untreated state.
(C) from about 0.3 to about 1 part by weight based upon (A) of a cyano-functional polyalkoxy silane having the formula: Leempol (U.S. Pat. No. 4,962,152), Inoue et al. (U.S. Pat. No. 4,623,693), Letoffe (U.S. Pat. No. 4,672,003), Hirai et al. (U.S. Pat. No. 4,891,393) and Progneaux and Trego (U.S. Pat. No. 4,585,821). None of these, however, shows auto adhesion to polycarbonate.
It is desirable to provide a curable RTV silicone composition containing a reinforcing inorganic filler, having a combination of properties including translucency, alkoxy cure, fast cure, and shelf stability coupled with excellent long term primerless adhesion to polycarbonate under dry and wet conditions.